Recently, a mobile communication system has evolved into a communication system based on orthogonal frequency-division multiplexing (OFDM) where the amplitude varies significantly unlike Global System for Mobile Communications (GSM). In particular, examples of the communication system based on OFDM include WiMAX and WLAN in addition to LTE.
However, an OFDM based transmitter has a limitation in that its power amplifier does not work efficiently due to overcome the limitation, a variety of transmission structures are proposed, one of which is a polar transmitter structure for separately transmitting a phase signal and an envelope signal to a power amplifier.
The polar transmitter has a structure where the phase signal may be applied to an input terminal of the power amplifier and the envelop signal may be connected to a power supply terminal of the power amplifier to output an original signal from an output terminal of the power amplifier. The power amplifier of the polar transmitter can always operate in a saturation region because of the envelope signal applied to the power supply terminal, thereby maximizing the power efficiency.
However, as the envelop curve has a larger range of variations, a lower level of voltage may be input to the power supply terminal of the power amplifier. Such low envelop voltage causes several limitations. First, efficiency in an envelope modulator for providing the envelope signal to the power amplifier is rapidly reduced when a level of the envelope signal is smaller than a specific level. Accordingly, the polar transmitter structure operates less efficiently in a period where an envelope level is small. Furthermore, if the envelope voltage decreases to a knee voltage of the power amplifier or lower, nonlinearity of the power amplifier may increase rapidly to reduce signal quality.